This invention pertains generally to image transfer technology. More particularly, this invention relates to clock independent pulse width modulation that enables generation of laser printer data at a desired frequency regardless of the operating frequency of the pulse width modulator.
When rendering images using laser printer technology, a latent image is created on a surface of an insulating, photo-conducting material usually in the form of a rotating drum by selectively exposing areas on the surface of the drum to light. In operation, laser printers print pages by applying black toner onto selected small regions of fixed size referred to as pixels. By placing toner in only a portion of a pixel region, it is possible to create the effect of shades of gray. One presently recognized technique for placing toner in only a portion of a pixel region uses pulse width modulation (PWM). However, there exists a need to improve presently available prior art techniques as described below.
Pursuant to laser printer technology, a latent image is created on a surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. For the case of a laser printer, the surface is in the form of a rotating drum. A difference in electrostatic charge density is created between areas on the surface depending on the degree to which such areas are exposed to light. A visible image is then developed on the drum using one or more types of electrostatic toner. For the case of black and white printing, a single, black toner is used. For the case of color printing, multiple different color toners are used. Each toner is selectively attracted onto the photoconductive surface of the drum either exposed or unexposed to light, depending on the relative electrostatic charges on the photoconductive surface, characteristics of the development toner, and the type of toner used. Depending on the particular implementation, the photoconductive surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
The developed image is then transferred from the drum surface onto a sheet of paper. More particularly, a transfer roller is imparted with an electrostatic charge that is opposite to that of the toner. The transfer roller is rotated in proximity with the photoconductive surface of the drum. The transfer roller pulls the toner from the photoconductive surface, transferring the toner onto a charged sheet of paper. The transferred toner maintains the pattern of the image that was developed on the photoconductive surface.
In operation, a laser printer scans a laser beam horizontally across the photosensitive, electrically charged drum. By modulating the laser beam via a pulse width modulator (PWM), resulting variations in charge will impart proportionate amounts of toner being deposited onto a sheet of paper.
More particularly, laser printers print pages of information onto individual sheets of paper by applying a particular toner, such as black toner, to selected small regions of fixed size referred to as pixels. By placing toner in only a portion of a pixel region, it is possible to create the effect of shades of gray. One technique for placing toner in only a portion of a pixel region uses a pulse width modulation (PWM) technique.
Laser printers are distinguished from other types of printers by their ability to place precise amounts of toner into very small regions of a page at relatively high speed. As a result, laser printers generate image quality that is far greater than most other types of printers. However, laser printers operate by scanning a photoconductive drum upon which a rendered image is held. This results in an intrinsic quantization of the image in the vertical direction of the page. Additionally, there exist limitations in circuitry that is used to modulate the horizontal scanning. These limitations result in quantization of the image so that a single cell, or pixel, is effectively formed. If pixels are made small enough, the quantization effects can be made imperceptibly small to the human eye. However, there are practical limits. First, the vertical quantization is limited by the scan rate and the speed with which the photoconductive drum is rotated. Secondly, horizontal quantization is limited by the ability to transfer data in serial form to the scanning laser. The horizontal quantization limits the number of transitions that modulate the scanning laser, thereby limiting the density of horizontal dots that are placed onto a printed page.
In an effort to increase the resolution capability of laser printers, various techniques have been used to increase the number of horizontal dot components of a laser video signal generated by a laser. Irrespective of the technique used to generate horizontal dot components, the laser needs to be phase locked to a single signal edge that is referred to as a beam detect. The beam detect provides a reference signal that indicates when the scanning laser begins to sweep across the photosensitive drum.
In operation, a pixel clock is phase locked to the beam detect signal. One technique uses a clock generator as described in U.S. Pat. Nos. 5,438,353 and 5,760,816 listing Applicant as the inventor, and describing such clock generators. Such U.S. Pat. Nos. 5,438,353 and 5,760,816 are herein incorporated by reference.
According to the above-referenced clock generator prior art technique, a clock generator uses a chain of delayed clocks in order to generate phase locked, variable phase video output signals. More particularly, the clock generator comprises a variation of a pulse width modulator (PWM). For example, U.S. Pat. No. 5,438,353 discloses a variable phase version clock generator. Additionally, U.S. Pat. No. 5,760,816 discloses a reduced clock domain version clock generator which is referred to as a phase adjusted pulse width modulator (PWM). Such U.S. Pat. Nos. 5,438,353 and 5,760,816 are herein incorporated by reference as indicating details presently understood in the art.
Improvement has been made to the clock generator technique described in the above-referenced patents. More particularly, it is desirable to generate horizontal dots within a single pixel clock time, thereby increasing the effective resolution of a printed page. Such a technique generates sub-pixels within a pixel dot clock cycle. This technique can be accomplished using a phase adjusted, or phase locked, pulse width modulator. See U.S. patent application Ser. No. 09/534,747, entitled xe2x80x9cA METHOD AND DEVICE FOR TIME SHIFTING TRANSITIONS IN AN IMAGING DEVICExe2x80x9d, filed Mar. 24, 2000, naming the inventors as Robert D. Morrison and Eugene A. Roylance, which application describes one technique using phase adjusted transition placement. Such U.S. patent application Ser. No. 09/534,747 is herein incorporated by reference. Such U.S. Patent Application provided a substantial improvement over previous prior art techniques disclosed in U.S. Pat. Nos. 5,438,353 and 5,760,816 wherein a clock generator of such previous prior art techniques introduces error in dot placement when using the clock generator on pixel boundaries. A standard phase locked loop generally cannot be used in such an environment because it is not possible to generate an error signal that would correct the pixel dot clock. More particularly, there is only one beam detect edge that the pixel dot clock can lock phase to, and that phase is required to be rigidly maintained for the length of a scanned line.
However, there is still a problem which results from implementing the above-referenced clock generators because the clock frequency that specifies the width of video pixels is usually different than the clock that is used to run the formatter processor. The clock used to run the formatter processor formats user data into a raster image that can modulate the laser. As a result, three problems occur. First, there are significant delays induced by the handshaked transfer of data from the processor to the video circuitry. The processor is running on a system clock, while the video circuitry is running on a video clock. Secondly, ASIC design and testing is significantly hampered by multiple clock domains that do not easily communicate with each other. The circuitry on one clock domain is completely asynchronous to circuitry in the other clock domain. Thus it is a requirement, for example, that there be two completely independent test vector sets. As a result, portions of the interface between the clock domains will not be well tested. Furthermore, there will be nearly a doubling of overhead in setting up the test. Thirdly, the nature of an asynchronous interface that is provided between clock domains and the nature of the current pulse width modulation (PWM) technology is such that if either clock domain frequency changes, the ASIC is often unable to correctly function due to the change in ratio of the frequencies. For example, if the printer scanner or system clock speeds change for any reason, the ASIC is unable to correctly function because the handshake between clock domains cannot always function in the new clock domains. This limits the ability to re-use the ASIC when a different printer engine is developed.
Even though it may be possible to move the system clock frequency in order to match the video clock frequency, it would still be necessary to provide two separate clocks. A system clock has a frequency that is defined by processor performance, whereas a video clock has a frequency that is defined by the print engine. Although it may be possible to run such clocks at the same frequency, rarely will it be possible to run the system clock at the same frequency as the video clock, or even to run such clocks at frequencies wherein one is a multiple of the other (wherein scaling can be used to adjust the frequencies where they are multiples of one another). Accordingly, a first clock is needed to step the processor and memory accesses, while another clock is needed to provide a reference that proscribes the width of a video frequency. The resulting two frequencies would almost never be derivable from each other since the processor memory access performance drives the system clock speed, while the physical mechanics of a laser printer engine drives the video clock speed. Accordingly, the video clock generates a fixed number of delayed clocks within a pixel in order to permit sub-pixel pulse width modulation. However, the two frequencies are not derivable from one another, and are not functionally dependent thereupon.
Therefore, there exists a need for improved techniques for implementing pulse width modulations (PWMs) with lasers on image forming devices.
An apparatus and method, in the form of a circuit, are provided for modifying and adding functionality to a pulse width modulator (PWM) in order to enable the generation of video data at any frequency, regardless of the operating frequency of the pulse width modulator (PWM). For the case where a pulse width modulator (PWM) is clocked at a system processor frequency, an entire associated ASIC will have a single clock, and will be able to issue video data in order to modulate a laser at any desired video frequency. Because the video frequency is no longer fixed, for example, with a crystal oscillator, but can now be programmed into the ASIC, it is potentially possible to use the same ASIC or even the same formatter with multiple printer engines with different video frequency requirements. Accordingly, ASIC development (including costs and development time) has the potential to be made independent of a particular printer engine development path.
According to one aspect, a system is provided for enabling a pulse width modulator to render video data for a laser at a frequency independent of the operating frequency of the pulse width modulator. The system includes phase measuring circuitry, edge output determining circuitry, edge location circuitry, and a summer. The phase measuring circuitry is operative to detect a phase offset on a scan line for a laser. The edge output determining circuitry is operative to determine which system clock cycle to output an edge on the system clock to act as if it were on a video clock. The edge location circuitry is operative to locate placement of an edge for a given pixel. The summer communicates with the phase measuring circuitry, the edge output determining circuitry, and the edge location circuitry. The summer is operative to combine values from the phase measuring circuitry, the edge output determining circuitry, and the edge location circuitry to generate a desired video signal.
According to another aspect, a clock-independent pulse width modulator includes a pulse width modulator, a system clock, circuitry, and a summer. The pulse width modulator is operative to generate video data for rendering an image via a laser. The system clock is operative to generate a system clock signal. The circuitry includes phase offset measuring circuitry, edge output determining circuitry, and edge location circuitry. The phase offset measuring circuitry is operative to detect a phase offset on a scan line of a laser. The edge output determining circuitry is operative to determine which system clock cycle to output an edge on the system clock to act as if it were on a video clock. The edge location circuitry is operative to locate placement of an edge for a given pixel. The summer communicates with the phase measuring circuitry, the edge output determining circuitry, and the edge location circuitry. The summer is operative to combine values from the phase offset measuring circuitry, the edge output determining circuitry, and the edge location circuitry to generate a desired video signal.
According to yet another aspect, a method is provided for rendering an image onto an image transfer surface. The method includes: providing a laser, a pulse width modulator, and a system clock, the laser configured to generate an optical scan path onto an image transfer surface in response to video data that is received from the pulse width modulator; detecting a phase offset on a scan line of a laser; determining on which system clock cycle to output an edge such that the system clock is configured like a video clock; locating placement of an edge for a given pixel; and combining values from the detected phase offset, the edge output, and the edge location to generate a desired video signal.